A variety of structures have heretofore been employed as a means for locking down the plunger of a dispensing pump in a fully depressed position during periods of non-use. For example, co-pending application Ser. No. 06/207,892 filed Nov. 18, 1980, in the names of Magers, et al. and assigned to the assignee of the present invention, now U.S. Pat. No. 4,369,899, discloses a "bayonet" type lock down arrangement which has proven to be quite acceptable. The bayonet lock of the application is placed in its locking mode by depressing the plunger fully until locking lugs on the plunger are aligned with receiving notches in the collar through which the plunger reciprocates, whereupon the plunger is rotated slightly relative to the collar to slip the lugs underneath overhead shoulders or ledges of the collar. In this manner, axial movement of the plunger in either direction is precluded, so long as the plunger is not rotated out of its locked mode.